\Header{grep}{Pattern Matching and Replacement}
\alias{gsub}{grep}
\alias{regexpr}{grep}
\alias{sub}{grep}
\keyword{character}{grep}
\keyword{utilities}{grep}
\begin{Description}\relax
\code{grep} searches for matches to \code{pattern} (its first
argument) within the character vector \code{x} (second
argument).  \code{regexpr} does too, but returns more detail in a
different format.

\code{sub} and \code{gsub} perform replacement of matches determined
by regular expression matching.\end{Description}
\begin{Usage}
\begin{verbatim}
grep(pattern, x, ignore.case = FALSE, extended = TRUE, perl = FALSE,
     value = FALSE, fixed = FALSE)
sub(pattern, replacement, x,
        ignore.case = FALSE, extended = TRUE, perl = FALSE)
gsub(pattern, replacement, x,
        ignore.case = FALSE, extended = TRUE, perl = FALSE)
regexpr(pattern, text,  extended = TRUE, perl = FALSE, fixed = FALSE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{pattern}] character string containing a regular expression (or
character string for \code{fixed = TRUE}) to be matched in the given
character vector.
\item[\code{x, text}] a character vector where matches are sought.
\item[\code{ignore.case}] if \code{FALSE}, the pattern matching is \emph{case
sensitive} and if \code{TRUE}, case is ignored during matching.
\item[\code{extended}] if \code{TRUE}, extended regular expression matching
is used, and if \code{FALSE} basic regular expressions are used.
\item[\code{perl}] logical. Should perl-compatible regexps be used if
available?  Has priority over \code{extended}.
\item[\code{value}] if \code{FALSE}, a vector containing the (\code{integer})
indices of the matches determined by \code{grep} is returned, and if
\code{TRUE}, a vector containing the matching elements themselves is
returned.
\item[\code{fixed}] logical.  If \code{TRUE}, \code{pattern} is a string to be
matched as is.  Overrides all other arguments.
\item[\code{replacement}] a replacement for matched pattern in \code{sub} and
\code{gsub}.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The two \code{*sub} functions differ only in that \code{sub} replaces
only the first occurrence of a \code{pattern} whereas \code{gsub}
replaces all occurrences.

For \code{regexpr} it is an error for \code{pattern} to be \code{NA},
otherwise \code{NA} is permitted and matches only itself.

The regular expressions used are those specified by POSIX 1003.2,
either extended or basic, depending on the value of the
\code{extended} argument, unless \code{perl = TRUE} when they are
those of PCRE,
\url{ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/}.\end{Details}
\begin{Value}
For \code{grep} a vector giving either the indices of the elements of
\code{x} that yielded a match or, if \code{value} is \code{TRUE}, the
matched elements.

For \code{sub} and \code{gsub} a character vector of the same length
as the original.

For \code{regexpr} an integer vector of the same length as \code{text}
giving the starting position of the first match, or \eqn{-1}{} if there
is none, with attribute \code{"match.length"} giving the length of the
matched text (or \eqn{-1}{} for no match).\end{Value}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole (\code{grep})\end{References}
\begin{SeeAlso}\relax
\code{\Link{agrep}} for approximate matching.

\code{\Link{tolower}}, \code{\Link{toupper}} and \code{\Link{chartr}}
for character translations.
\code{\Link{charmatch}}, \code{\Link{pmatch}}, \code{\Link{match}}.
\code{\Link{apropos}} uses regexps and has nice examples.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
grep("[a-z]", letters)

txt <- c("arm","foot","lefroo", "bafoobar")
if(any(i <- grep("foo",txt)))
   cat("'foo' appears at least once in\n\t",txt,"\n")
i # 2 and 4
txt[i]

## Double all 'a' or 'b's;  "\" must be escaped, i.e., 'doubled'
gsub("([ab])", "\\1_\\1_", "abc and ABC")

txt <- c("The", "licenses", "for", "most", "software", "are",
  "designed", "to", "take", "away", "your", "freedom",
  "to", "share", "and", "change", "it.",
   "", "By", "contrast,", "the", "GNU", "General", "Public", "License",
   "is", "intended", "to", "guarantee", "your", "freedom", "to",
   "share", "and", "change", "free", "software", "--",
   "to", "make", "sure", "the", "software", "is",
   "free", "for", "all", "its", "users")
( i <- grep("[gu]", txt) ) # indices
stopifnot( txt[i] == grep("[gu]", txt, value = TRUE) )
(ot <- sub("[b-e]",".", txt))
txt[ot != gsub("[b-e]",".", txt)]#- gsub does "global" substitution

txt[gsub("g","#", txt) !=
    gsub("g","#", txt, ignore.case = TRUE)] # the "G" words

regexpr("en", txt)

## trim trailing white space
str = 'Now is the time      '
sub(' +$', '', str)  ## spaces only
sub('[[:space:]]+$', '', str) ## white space, POSIX-style
if(capabilities("PCRE"))
  sub('\\s+$', '', str, perl = TRUE) ## perl-style white space
\end{ExampleCode}
\end{Examples}

